1. Field of the Invention
The present invention relates to techniques for encrypting and decrypting data. More specifically, the present invention relates to a method and an apparatus for encrypting data and managing keys to facilitate making data permanently unreadable.
2. Related Art
To ensure data is not lost prematurely, it is common to create multiple backup copies of the data. However, it is also desirable in some cases to ensure that a file, once deleted, is not recoverable. This can be a rather complicated task if backup copies of the data are created and stored at different locations.
This problem can be solved by only storing the data in encrypted form. In this way, destroying the data is a somewhat easier problem because only the key must be deleted. However, long-term user keys can, over time, be obtained by an adversary through compromise or coercion. To remedy this problem, it is possible for keys to be kept in tamper-resistant smart cards, in which case it is not feasible to covertly discover the keys. To delete the data, the user need only destroy the smart card. However, it is expensive to require every user to have a smart card and every computer to have a smart card reader.
A more-sophisticated technique for managing secret keys (developed by a company called “Disappearing Inc.”) uses a special server called an “ephemerizer” whose job it is to create and destroy keys. A nice property of this technique is that the ephemerizer can be built so it does not see any data. However, the ephemerizer must create and store a key for every ephemerally-created message. This can involve storing a large amount of data if the system is used to encrypt many messages. (Also see a related system disclosed in U.S. Pat. No. 6,363,480, entitled “Ephemeral Decryptability,” by inventor Radia J. Perlman.)
This storage-space problem can be alleviated by modifying the ephemerizer so that it only maintains one key per expiration time, and having that key used across many users and many files. (See U.S. patent application Ser. No. 10/959,928, filed on 5 Oct. 2004, entitled “Method and Apparatus for Using Secret Keys to Make Data Permanently Unreadable,” by inventor Radia J. Perlman. This application is hereby incorporated by reference to disclose how an ephemerizer is designed and operates.)
However, this approach does not provide a scalable solution when a file is deleted on demand, since if a key is used for multiple files, that key cannot be deleted without making all other files encrypted with that key unreadable.
Hence, what is needed is a method and an apparatus that facilitates making data disappear without the above-described problems.